Wireless communication systems, such as analog and digital cellular communication systems, personal communication systems (PCS) and other similar wireless communication systems, provide a great deal of freedom to their users. A wireless communication system user is almost always in touch whether on the road or at the home or office. And, in spite of the complexity underlying the wireless communication system, to the user the system is as easy to use as dialing a phone number.
Sometimes in wireless communication systems, a user will be unable to place or receive a call, or an ongoing call will be unexpectedly disconnected. One has to remember that at least a portion of the wireless communication system is a radio frequency (RF) link between a remote or mobile user and the system. There are a number of factors which influence how and why a call may not be completed or is disconnected. One cause lies with the limited number of radio frequency resources available for a given service area. Radio frequency resources are limited based in part on the allocation of radio frequency spectrum for particular applications. For example, television broadcasts are allowed a certain portion of the radio frequency spectrum while wireless communication networks are allocated another portion of the radio frequency spectrum. The allocations are such that operation of one system does not create interference in the other system due to radio frequency reuse.
However, certain wireless communication system architectures, such as those based upon the interim standard, IS-95-A for a code division multiple access (CDMA) wireless communication network, overcome radio frequency resource limitations by offering an ability to use common radio frequencies for multiple users. Capacity problems, or limitations on a user's ability to access and use these systems, may remain as a result of a limitation on the number of users the system can process. The solution here, of course, is to expand the capacity of the system. Unfortunately, present system architectures do not provide for ready expansion of system capacity.
For example, one would think simply adding additional equipment to handle the additional users would solve the capacity problems. However, the addition of equipment, and particularly in CDMA based wireless communication systems, creates certain system performance problems. For example, as additional system capacity in the form of additional base transceiver stations (BTSs), base station controllers (BSCs) and mobile switching centers (MSCs) is added, the aggregate area covered by each MSC/BSC/BTSs group becomes smaller. This means more seams, i.e., more interfaces between coverage areas.
Additional seams in the communication system may mean more frequent handoffs and particularly more "hard" handoffs. Additional seams may also require additional processing resource utilization and may result in increased voice delay due to traffic interconnect and increased latency on execution of call processing procedures. Seams require additional system engineering and in many cases lead to decreased call quality.
CDMA communication systems employ a process known as "soft" or "softer" handoff to reduce call quality degradation resulting from hard handoffs by permitting the mobile station to communicate with several BTSs. Soft handoff is advantageously employed when the mobile station is moving from an area covered by one BTS to an area covered by another BTS. In soft handoff, the mobile station is always in active communication with at least one BTS even as it moves through the system from BTS coverage area to BTS coverage area. This results from each of the BTSs operating under the control of a particular BSC using a common set of radio frequencies.
Hard handoff seams almost always have a negative effect on call quality and as such are avoided as much as possible. A hard handoff may occur as a mobile station moves from a geographic area served by a first BSC to a geographic area served by a second BSC. Handoff between a BTS serviced by the first BSC and a BTS serviced by the second BSC requires that a communication link be established in the second BSC through the appropriate BTS associated with the second BSC. When handoff is necessary, i.e., as the mobile station moves out of the service area of the first BTS into the area serviced by the second BTS, the mobile station must reestablish the call through the second BSC. Communication with multiple BTSs is generally not possible in this mode. More importantly, communication between the mobile station and the BTS/BSC is normally momentarily disrupted. Thus, one will appreciate that adding capacity in the form of additional MSCs, BSCs and BTSs will increase the number of seams, and particularly, may result in an increase in hard handoff seams and the associated disruption in service.
Therefore, there is a need for a wireless communication system architecture which is easily and readily scalable, expandable, as the number of users of the system expands. More importantly, such system expansion should be provided at minimum cost and without degrading system performance.